An image processing system that detects a traffic indicator from image data on a scene ahead of a vehicle is conventionally known (Japanese Patent Application Publication No. 2007-241469). Japanese Patent Application Publication No. 2007-241469 detects the position and attitude of a vehicle to thereby predict the position of a traffic indicator, determines an image processing area within image data based on the predicted position, and detects the traffic indicator from the image processing area. The image processing system thus decreases the image processing load.
The image processing system of Japanese Patent Application Publication No. 2007-241469, however, does not consider error in the detected position and attitude of the vehicle when determining the image processing area. The error is affected greatly by the surrounding situation of the vehicle. If the error is large, the traffic indicator will be outside the image processing area and therefore will be undetectable. On the other hand, setting an excessively large image processing area increases the possibility of erroneously detecting an object other than the traffic indicator.